It is important to provide either a local and/or remote copying capability of data for real-time backup protection of data stored in a data processing installation, such as in peripheral data-storage. Backing up or copying to a remote data center protects against physical disasters not protected by a local_ backup copy. This automatic data copying is commonly referred to as remote duplexing or remote data copying. The prior art is replete with methods and means for backing up data for recovery and archiving purposes. An example of such a system can be found in U.S. Pat. No. 5,555,371 to Duyanovich et al.
Although backup copying techniques are frequently utilized to preserve and protect data, the use of more than one remote or secondary backup storage site is cost prohibitive, particularly for data-storage systems that store large amounts of data. Accordingly, there is a need to provide additional cost efficient data preservation techniques.
A more cost efficient data preservation technique is to perform a snapshot copy. A snapshot copy is an image of the physical location or directory (i.e., the physical address, size, etc.) of data in the data-storage unit (which may include a plurality of volumes). The snapshot copy can be used either to restore an image of a protected data-storage unit if the data-storage unit should fail, or to compare historical snapshots of data from a specified prior time period to a current version of the data. The advantages of snapshot copying is that a snapshot copy of any data file, volume, etc. can be accessed in a matter of seconds, and the snapshot copy allows the data file or volume to be restored to an original version at the instance of time the snapshot copy was taken.
One problem with snapshot copying is that access to a volume being snapshot copied is suspended until the snapshot operation is completed. This prevents the volume from being accessed or altered at the same time as a snapshot operation is being performed. Another problem is that snapshot copying requires storage space or addressing to be allocated for the snapshot copy. This is particularly problematic in data-storage systems with limited storage capacity or limited addresses. Accordingly, users are limited in the number of snapshot operations that can be performed without adversely affecting the efficiency, accessibility and storage capacity of the data-storage system.
There is a need for a method and apparatus to snapshot copy a volume on a primary storage subsystem without requiring storage space to be allocated at the primary storage subsystem for the snapshot copy. There is also a need to provide a method and apparatus to snapshot copy a volume on a primary storage subsystem, while simultaneously allowing the volume being snapshot copied to be accessed.
Accordingly, it is an object of the present invention to provide a method and apparatus for performing an equivalent of a snapshot copy operation of a volume of a primary data-storage subsystem without requiring storage space to be allocated at the primary data-storage.
It is a further object of the present invention to provide a method and apparatus for performing an equivalent of a snapshot copy of a volume of a primary data-storage subsystem, while allowing that volume to be accessed by a host device, during the performance of the snapshot copy operation.
Another object of the present invention is to provide a method and apparatus which employs a secondary data-storage subsystem to perform the equivalent of a snapshot copy of a volume of a primary data-storage subsystem.
It is also an object of the present invention to provide a method and apparatus for performing a snapshot copy of a volume of a primary data-storage device which can be integrated into pre-existing data-storage systems that already perform remote dual copying of volumes.